Quartus II v6.1 crashes my PC

No matter how bad of a coding mistake, Quartus should always crash nicely within the windows environment(internal error, maybe worst case is Quartus "disapearing"). The fact that it brings down your system is worrisome. I've seen this once before(someone locally in the office), and his problem ended up being a RAM issue. (He thought it was Quartus, since that was the only thing that caused the problem, but it was more because Quartus was the only thing that used so much RAM it encountered the bad area.) I know you've already run some tests, but that is the only thing I could think of. And something that was truly just a bad line of code in Quartus should be repeatable, i.e. every time you compile it, even on different machines, you get the same problem. That doesn't seem to be the case here.

Can you upgrade to the latest version of Quartus? I really don't think that's the problem, but at least it's another data point. Do you work with someone who has another machine and could run some of your compiles for a few days(not as a workaround, but to see if it is machine specific). Just some thoughts, but good luck.